Vaccination has become a standard procedure for the prevention of numerous infectious diseases. The application of vaccines to other diseases, such as cancer, is now an attractive possibility due to advances in molecular engineering and a better understanding of tumor immunology.
Cancer is one of the leading causes of mortality worldwide. Despite an abundance of cancer-related research, conventional therapies that combine surgery, radiation, and chemotherapy, often fail to effectively treat established cancers. Reliable methods of prevention also remain unavailable.
Cancer typically involves the malfunction of genes that contribute to the regulation of the cell cycle or cell proliferation, such as growth factors and their receptors, oncogenes, and tumor suppressor genes. The products of many of these genes are expressed on the surface of a wide variety of tumor cells and, hence, are designated tumor-associated antigens (TAAs). The introduction of genes encoding TAAs directly into a subject has been shown to generate a protective immune response against the TAA in many experimental models, making these molecules a target for vaccine therapy. However, because many of these gene products are also expressed in normal cells, albeit at lower levels, many immunological therapies targeting TAAs have proven ineffective due to self-tolerance.
Genes coding for several tumor-associated antigens (TAA) have been isolated, characterized and inserted in genetic vectors such as plasmid DNA and viral vectors. One tumor-associated antigen that has been implicated in the pathogenesis of cancer is telomerase (TERT).
Telomerase is a DNA polymerase that normally functions in maintaining telomere length at the ends of chromosomes. During normal cell growth, an RNA primer attaches to the 5′ end of DNA and initiates replication. Upon removal of the RNA primer, a gap in length is introduced to the resulting daughter strand of DNA. Thus, replication of a linear strand of DNA with conventional polymerases leads to shortening of telomere length in each progressive round of replication. Such shortening of telomere length is responsible for cellular senescence or aging of normal human somatic cells.
Telomerase is a ribonucleoprotein comprising an RNA component and a catalytic protein component (telomerase reverse transcriptase). The catalytic component of human telomerase was described by Meyerson et al. (Cell 1197: 785-95 (1990) and Nakamura et al. (Science 277: 955-59 (1997)). The TERT enzyme uses its RNA component as a template for telomere DNA synthesis, thus allowing telomeres to maintain their length over successive generations of cell growth. Such maintenance of telomere length over numerous proliferative cycles allows a cell to escape normal senescence and become immortal, allowing a tumor to grow and metastasize over great lengths of time. Because telomerase confers replicative immortality to cells, telomerase activity has been detected in cancerous cell lines and a diverse range of tumor types (Kim et al. Science 266: 2011-15 (1994)). Conversely, telomerase is inactive or only transiently expressed at low levels in normal human tissues and normal somatic cell cultures. The combination of telomerase over-expression in most cancer types as well as low or absent expression in normal cells makes TERT a target for therapy and/or prophylaxis of diseases associated with aberrant cellular proliferation such as cancer.
The development of a telomerase-specific vaccine is now possible because the catalytic and RNA components of telomerase have been cloned and characterized (see, e.g., U.S. Pat. No. 6,166,178). However, the development and commercialization of many vaccines have been hindered by difficulties associated with obtaining high expression levels of the desired immunogen in successfully transformed host organisms. The development of efficacious DNA-based vaccines has also been hindered by an inability to generate an immune response of sufficient magnitude in treated individuals to lead to tumor regression in a clinical setting. Therefore, despite the identification of the wild-type nucleotide sequences encoding telomerase proteins described above, it would be highly desirable to develop a vaccine which is capable of eliciting an enhanced TERT-specific immune response relative to a wild-type full-length TERT cDNA, when delivered to a mammal. It would also be desirable to develop methods for treating or preventing TERT-associated cancers which utilize nucleic acid molecules or proteins that safely and effectively potentiate a TERT-specific immune response.